Hardy trout gorges, starves itself to survive


SEATTLE — Scientists say evolution is partly to blame for the obesity epidemic sweeping the globe today. In a world where the next meal was unpredictable, our ancestors were hard-wired to gorge when the opportunity presented itself.

But researchers at the University of Washington have discovered a type of trout that puts ancient humans to shame when it comes to coping with a feast-or-famine environment.

Living in a remote watershed on the Gulf of Alaska where food is abundant for only five weeks of the year, the fish survive by expanding and shrinking their digestive tracts in a way that modern humans trying to lose weight might envy.

The results could help wildlife managers as they attempt to save bull trout, a related species that is threatened in the Pacific Northwest, said Jonathan Armstrong, co-author of a study published Wednesday in the Journal of Animal Ecology.

Working in Alaska’s Chignik watershed, Armstrong and fellow fisheries doctoral student Morgan Bond focused on a type of trout called Dolly Varden.

The Chignik watershed is famous for its sockeye salmon run. Up to a half-million of the crimson fish return like clockwork in late July to spawn, turning the river red. For Dolly Varden, the sockeye’s arrival is like a dinner bell. The trout eat up to half a pound a day of salmon eggs from the spawning frenzy.

But by late August, the rivers and lakes in the Chignik area are cold and barren of insects or other prey for the Dolly Varden, Bond said. He and Armstrong wondered how the fish could survive under such extreme conditions.

The pair surveyed the river in spring, before the sockeye returned, and found it populated with very skinny Dolly Varden. When they cut open some of the fish, they found their digestive systems were tiny, too.

Fish captured after the salmon-egg buffet weighed up to 50 percent more, and some parts of their digestive tracts were four times bigger.

The fish were bulking up their digestive systems to capitalize on the banquet.

Maintaining a large digestive system is very costly; it can use up to 30 percent of a fish’s energy, so Dolly Varden essentially jettison their digestive machinery when they no longer need it and live off their fat reserves for 11 months or so in a kind of stasis.